Drought damages trees' ability to store carbon

How long-term forest health is compromised years after dry-spells.

Aspen are the rock stars of the tree world. They have a bold fashion sense, gilding the mountains in gold each fall. And they engage in risky behavior. In the competitive world of plant biology, their strategy is to grow fast and die young. Juniper, which grows ploddingly slowly, invests much of its carbon in building strong vascular tissue; aspen instead puts its carbon toward growing tall quickly.

Yet the two trees adopt the same strategy when drought hits: Unlike pines, they leave their stomata open. Those tiny pores on their leaves allow the trees to take up carbon dioxide from the atmosphere and photosynthesize. But the atmosphere demands water in return, which escapes through the tree's stomata. During drought, the process creates extra tension in the plant's tissue, since the trees have less water to give and the parched atmosphere is especially thirsty.

This is when juniper's investments in building tough tissue pays off; the tubes that transport water through the tree can withstand an increase in tension. Aspen, on the other hand, become vulnerable. Their tubes are more likely to collapse – like a straw does when you're trying to suck the final drops of soda, but just getting air instead – which can eventually lead to the tree's death.

A few years ago, Bill Anderegg, a forest ecologist and now an assistant professor at the University of Utah, figured out that this was what was killing aspen in his native Colorado. (Around 2004, aspen throughout the Rocky Mountains began mysteriously dying in droves, a phenomenon that was dubbed "sudden aspen decline.") But he noticed something else curious about their pattern of death. There seemed to be a lag between the drought and its consequences. Tree growth would slow for a few years, but often they wouldn't start to die until three to eight years after the drought.

Dead aspen in Colorado, courtesy Leander Anderegg.

"There was an interesting paradox of trees being stressed and dying from drought in completely wet soils," Anderegg says. "Which prompted me to ask the question: How widespread are these legacy effects?"

It turned out they were quite widespread, according to the results of a study Anderegg authored in the journal Science this week. Using data from a global tree ring archive, Anderegg measured tree growth following severe drought at 1,338 sites, primarily in the Northern Hemisphere and outside the tropics. Growth is a good proxy for forest health, Anderegg explains, and it's also the primary way trees store carbon. That's important because, globally, forests take up about a quarter of anthropogenic carbon emissions. Their continued ability to sequester carbon at that level is critical to blunting the most severe effects of climate change.

With a few exceptions, tree growth slowed for two to four years after severe drought, with the most enduring effects appearing in arid environments – another indication of the troubles Southwestern forests are likely to face as the climate warms.

"It does seem like species that took more risks during drought seemed to recover more slowly," he says, like aspen, which don't close their stomata and conserve water. "Which does indicate that there's some role of this damage to their hydraulic systems that could slow their growth in subsequent years."

Scientific models of the global carbon cycle – which are important for projecting climate change – don't account for this slow-down in growth. "The models assume there is no lag, so as soon as climate is better, so is growth," says Nate McDowell, who researches the physiology of tree death at Los Alamos National Lab in New Mexico. That means that models may overestimate the ability of ecosystems to store carbon – and underestimate the severity of future climate change.

If droughts do become more frequent and severe, Anderegg says, as climate models predict, "this suggests that more forests are going to spend more and more of their time recovering, and become less good at taking up carbon."Anderegg estimates that in Southwestern forests, the lag could amount to a 3 percent reduction in their carbon storage over a century. That may not sound like much, but when it comes to squirreling away the emissions we stubbornly keep spewing, we need all the help we can get.

Plus, the meaningfulness of such numbers is a matter of scale, notes Adrian Das, a forest ecologist with the U.S. Geological Survey in the Southern Sierra Nevada. While we don't know what the precise effect of that reduced carbon storage might be, locally or globally, "these changes can translate into really large absolute numbers," he says. "Three percent is not very much if it’s five trees. It means something different if it’s thousands of trees."

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